This application has priority benefit of Japanese Patent Application No. 00/258745, filed on Aug. 29, 2000.
(1) Field of the Invention
The present invention relates to a coating film-forming method, more particularly to a coating film-forming method which comprises coating a cationic electrodeposition coating composition containing an electrically conducting agent onto a metal substrate such as an automobile body, followed by curing and drying, coating an anionic electrodeposition coating composition onto a cured coating film to form a non-cured coating film, coating a topcoating composition onto the non-cured coating film to form a non-cured topcoating film, and heat curing and drying a resulting laminated film simultaneously.
The present invention also relates to a coating film-forming method, wherein a coating color of the anionic electrodeposition coating film is the same series of chromatic color in accordance with a hue represented by Mansell color system as a coating color from a topcoating composition coated thereafter.
(2) Description of Background Art
It is known in the art that the cationic electrodeposition used as a paimer coating for an automobile body is applied to one coat electrodeposition coating as well as two coat electrodeposition coating (W coat electrodeposition coating), wherein a first electrodeposition is coated to form a first electrodeposition coating film, followed by coating thereonto a second electrodeposition coating composition having a function different from the first electrodeposition coating film (see Japanese Patent Application Laid-Open Nos. 41994/95, 324292/97, 8291/98, etc.).
For the purpose of satisfying a recent demand of more and more improving an electrodeposition coating film in corrosion resistance and weather resistance, Japanese Patent Application No. 192439/99 teaches a coating film-forming method which comprises coating a cationic electrodeposition coating composition containing an electrically conducting agent to form a cured dry film, followed by coating an anionic electrodeposition coating composition thereonto a form a cured dry film showing good weather resistance.
On the other hand, for the purpose of reducing cost and improving productivity in the automobile body coating, attempts have been made on omitting a curing step by a wet-on-wet coating, omitting an intercoat coating step, or on reducing a coating film thickness. However, for the purpose of satisfying recent demands by users on good finished appearance, whiter appearance, improved chipping resistance, etc., attempts to reduce a pigment concentration in order to ensure opacifying properties and chipping resistance on omitting an intercoat coating and coating film therefrom, and to improve finished appearance by increasing a coating film thickness or the like have been made. Applications of a water-based intercoating or topcoating are considered as an effective means to solve environmental coating problems.
The above case, however, has produced problems required to be solved, for example, reduction in opacifying properties due to reduction in a pigment concentration, increase in cost due to increase in the coating film thickness, and particularly reduction in finished properties due to sagging of a water based topcoating film on a vertical coating along a press line of a door part or around a keyhole part.
For the purpose of solving the above problems, the present inventers made intensive studies to find out a coating film-forming method according to a two coats-one bake electrodeposition coating (hereinafter may be referred to a W-coat electrodeposition coating), which method comprises coating a cationic electrodeposition coating composition containing an electrically conducting agent, heat curing to form a calionic electrodeposition coating film having a volume resistivity of 1012 xcexa9xc2x7cm or less, coating an anionic electrodeposition coating composition to form a non-cured anionic electrodeposition coating film, coating thereonto a topcoating composition to form a non-cured topcoating film, and heat curing the resulting laminated film simultaneously.
Preferably, the above coating film-forming method comprises coating an anionic electrodeposition coating composition such that a coating film color according to the anionic electrodeposition coating is the same series of chromatic color in accordance with a hue represented by Mansell color system as that from a topcoating composition to be coated thereafter, or white as an achromatic color to form a non-cured anionic electrodeposition coating film, coating the topcoating composition to form a non-cured topcoating film, and heat curing the resulting laminated film simultaneously.
The wet-on-wet coating of the water based topcoating composition onto the non-cured anionic electrodeposition coating film makes it possible to achieve improvements in finished properties, opacifying properties and anti-chipping properties even in the absence of an intercoating step, and further makes it possible to obtain good sagging resistance, resulting in accomplishing the present invention.
That is, an object of the present invention is to provide a coating film-forming method which is capable of achieving improvements in finished properties, opacifying properties and anti-chipping properties even in the absence of an intercoating step, and capable of obtaining good sagging resistance on the wet-on-wet coating of the water based topcoating composition onto the non-cured anionic electrodeposition coating film.
The present invention provides a coating film-forming method comprising a first step which comprises coating a cationic electrodeposition coating composition (A) containing an electrically conducting agent so that a cured coating film formed therefrom can have a volume resistivity of 1012 xcexa9xc2x7cm or less onto a metal substrate selected from the group consisting of an automobile body and automobile parts, followed by washing with water and heat curing and drying to form a cured cationic electrodeposition coating film; a second step which comprises coating such an anionic electrodeposition coating composition (B) that a coating film formed therefrom has a chromatic color or a white color as an achromatic color onto the cured cationic electrodeposition coating film, followed by washing with water, and removing water by preheating or air blowing to form a non-cured anionic electrodeposition coating film; and a third step which comprises coating a topcoating composition (C) thereonto to form a non-cured topcoating film, and heat caring and drying a resulting laminated coating film simultaneously, preferably wherein the topcoating composition (C) is such a topcoating composition that a coating film formed therefrom has the same series of chromatic color in accordance with a hue represented by Mansell color system as the color of the anionic electrodeposition coating film or a white color as an achromatic color.
The present invention also relates to the above coating film-forming method, wherein the topcoating composition (C) is a water based solid color topcoating composition (a).
The present invention also relates to the above coating film-forming method, wherein the third step comprises coating a water based metallic base coat (b) to form a non-cured coating film, coating thereonto a clear top coat (c) to form a non-cured coating film, and heat curing and drying a resulting laminated coating film simultaneously.